Such coordinate measuring machines comprise at least one high accuracy axis for precisely moving a probe along said axes. Typically, several stages of movement are realized with carriages moving along precision ways on their axis element, for which accordingly a guide and a drive is needed. High precision measurements are taken with the probe and therefore, a crucial requirement to the supporting structure of the CMM comprising the axis elements is constant shape accuracy, and therefore, in particular, distortions of the structure in the area of the axes are to be avoided as they directly cause measurement errors.
Coordinate measuring machines conventionally use mechanical drives, such as ball screws or rack and pinions, for said carriages. Even though these conventional drives offer a good positioning performance, they also require lubrication and maintenance in order to ensure a good durability. That is why, for some time past, belt drives, particularly so called “omega” belt drives are used as they provide good positioning accuracy while also realizing good response time and low wear.
A belt drive for the axes of a coordinate measuring machine is for example known from U.S. Pat. No. 4,651,426. Therein is shown a drive motor which is centrally mounted on a bridge portion overlying the columns of a coordinate measuring machine and which drives a horizontal shaft which extends to each column and effects the rotation of a drive pinion which engages a gear belt which extends parallel to each column and is carried by the column. The drive includes a toothed belt or gear belt which is disposed parallel to the guiding ways for the columns and has its ends firmly clamped on the bridge and an active central portion which extends around a drive pinion which is tensioned on respective sides of the drive pinion by tension rolls. Due to the progress of the belt around the pulleys the drive is also called “omega” drive.
Omega belt drives known from prior art commonly make use of a flexible elastic belt which generates vibrations when being exposed to sudden stress resulting from a fast acceleration or deceleration of the omega carriage. These vibrations cause a loss of precision as they are carried over to the frame of the coordinate measuring machine. Since belts are usually arranged in the air or simply abutting on the structure, the belt is not prevented from pulsating and passing on the vibrations either to the structure directly or via the mounting of the belt.